Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, carbon monoxide (CO), nitrogen oxides, sulfur oxides, and particulate matter. Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such internal combustion engines. Many different techniques have been applied to exhaust systems to clean the exhaust gas before it is emitted to the atmosphere.
One proposed method for cleaning exhaust gases produced in lean burn applications utilizes a three-way catalyst (TWC) followed by a selective catalytic reduction (SCR) catalyst. This concept, as described in U.S. Pat. No. 8,216,521, is to operate lean for an extended period of time during which the CO and hydrocarbons in the engine exhaust gas stream will be primarily oxidized to CO2 and H2O over the TWC. Periodic rich events are used to produce ammonia (NH3) over the TWC. The NH3 is stored and used by the downstream SCR catalyst to selectively reduce the NOx generated during the lean phase. The duration of these rich events requires optimization to provide adequate quantities of NH3 to the SCR to meet NOx conversion targets. Unfortunately the consequence of longer rich events is the generation of excess CO in the exhaust. It is critical to maintain high conversions of hydrocarbons, NOx, and CO during both lean and rich operating phases. As rich pulse times become longer to optimize the NH3 generation, CO becomes the most difficult species to control using only a TWC.
It is desirable to develop new and improved catalysts and methods that enable longer rich events while maintaining a high level of CO conversion. We have discovered a new CO slip catalyst that is capable of converting a large concentration of CO generated during rich operation.